Yajing Ling

The accessibility of nitrogen sites makes a difference in selective CO2 adsorption of a family of isostructural metal-organic frameworks

By using three rigid diisophthalate organic linkers incorporating different numbers and orientations of Lewis basic nitrogen atoms into the spacers between two terminal isophthalate moieties, namely, 5,5′-(quinoline-5,8-diyl)-diisophthalate, 5,5′-(isoquinoline-5,8-diyl)-diisophthalate, and 5,5′-(quinoxaline-5,8-diyl)-diisophthalate, a family of isostructural copper-based metal–organic frameworks, ZJNU-43, ZJNU-44 and ZJNU-45, were successfully solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. The three MOFs, after activation, exhibited almost the same porosities but distinctly different CO2 adsorption properties. At room temperature and 1 atm, the adsorption capacities for CO2 reached 103, 116 and 107 cm3 (STP) g−1 for ZJNU-43a, ZJNU-44a and ZJNU-45a, respectively. Furthermore, Ideal Adsorbed Solution Theory (IAST) and simulated breakthrough analyses indicated that ZJNU-44a bearing much more easily accessible nitrogen sites is the best among the three MOFs for the separation of the following two binary gas mixtures at 296 K, i.e., 50/50 CO2/CH4 and 15/85 CO2/N2 gas mixtures, indicating that the accessibility of nitrogen sites plays a much more crucial role, which is further confirmed by comprehensive quantum chemical calculations. The work demonstrates that the CO2 adsorption properties of MOFs depend not only on the number of Lewis basic nitrogen sites but also more importantly on their accessibility.

A porous lanthanide metal–organic framework based on a flexible cyclotriphosphazene-functionalized hexacarboxylate exhibiting selective gas adsorption

Design and synthesis of robust porous lanthanide-based metal–organic frameworks (Ln-MOFs) from flexible organic ligands is currently a formidable task to chemists. In this work, a porous Ln-MOF based on a flexible cyclotriphosphazene-functionalized organic ligand, hexakis(4-carboxylatephenoxy) cyclotriphosphazene, has been solvothermally synthesized. Single-crystal X-ray diffraction analyses show that the compound exhibits a three-dimensional structure built from rod-shaped secondary building units which are linked to each other through the organic ligands to form open frameworks with rectangular channels along the crystallographic a direction. Remarkably, although the flexible ligand was used, the Ln-MOF material after desolvation exhibited permanent porosity which has been established by various gas adsorption isotherms, displaying selective adsorption of C2 hydrocarbons over CH4 at room temperature. This work presents a rare example of a permanently porous Ln-MOF based on a flexible ligand exhibiting selective gas adsorption behaviours.

A new MOF-5 homologue for selective separation of methane from C2 hydrocarbons at room temperature

A new MOF-5 homologue compound UTSA-10 has been obtained under solvothermal conditions from a mixture of Zn(NO3)2⋅6H2O and commercially available linker, 2-methylfumaric acid, in N,N-dimethylformamide. The moderate surface area and suitable pore sizes enable the activated UTSA-10a to separate methane from C2 hydrocarbons at room temperature.

A metal–organic framework based on cyclotriphosphazene-functionalized hexacarboxylate for selective adsorption of CO2 and C2H6 over CH4 at room temperature

By using a flexible cyclotriphosphazene-functionalized hexacarboxylate, a novel metal–organic framework (ZJNU-60) has been solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. ZJNU-60 consists of dicopper paddlewheel secondary building units interconnected by organic linkers to form a bilayer with a rectangular channel running along the b axis. Upon desolvation, the framework underwent severe distortion and pore contraction, but the permanent porosity has been established as shown in C2H6, CO2 and CH4 gas isotherms. Furthermore, the activated ZJNU-60a displayed impressive adsorption selectivities toward C2H6 and CO2 over CH4, indicating the potential for the separation of CO2 and C2H6 from CH4. This is the first example of a MOF constructed from a flexible cyclotriphosphazene-functionalized hexacarboxylate for selective gas separation.